Imaging radiometers are used for many applications, such as aviation, including airport and aircraft safety and all-weather vision, medical and plasma diagnostics, non-destructive testing for voids and delaminations in composite materials, remote sensing of agricultural and environmental conditions, and a wide variety of defense, security, and law enforcement purposes.
An imaging radiometer measures the power of electromagnetic radiation or brightness temperatures emitted from a segment of a remote object, for example, a radio receiver that measures the average power of electromagnetic radiation including noise emitted from an object in a defined frequency range. Radiated power from a segment of a remote object contains information regarding the size, shape, and material composition of the object. Radiated object power is proportional to temperature and is commonly referred to as scene temperature. A radiometer captures scene temperatures through a signal chain designed to detect the typically weak radiated object power. The signal chain comprises three functions including: an aperture which collects electromagnetic energy, signal amplifiers which increase the received object power level, and a detector which transforms object power to either a voltage or current which are convenient for signal processing.
An imaging radiometer provides for a variety of different applications that deal with variation of the input power to detect objects or processes, for example, in target detection and medical imaging. Since the power of the input signal is typically substantially small, radiometers often employ signal comparison techniques, such as Dicke-switching to allow measurement of signal levels collected below the noise level of the signal chain. Accordingly, such radiometers also minimize (or neutralize) their gain variations by measuring the difference in signals between the signal chain input and a reference signal input.
A Dicke-switched radiometer is a radiometer that includes a Dicke switch that switches the input of the radiometer between a signal channel/chain and a reference channel/chain, for example, noise generated by a resistor.
FIG. 1 shows a conventional Dicke-switched radiometer. As shown, the Dicke-switched radiometer 100 employs a white noise resistive reference load 102 and a first (two-) stage tow noise amplification 104 as its reference channel. As the signal channel, the Dicke-switched radiometer 100 may include an antenna 108 for receiving an input signal and a second (two-) stage low noise amplification 106. The reference channel uses the calibrating resistive noise source 102 to modulate the signal captured by the antenna above the time-dependent noise (1/f noise) of the detector, low-noise amplifiers, and analog electronics. This modulation also assists in the removal of electronic DC offsets from each low noise amplifier (LNA) stage.
Dicke switch 110 rapidly switches the input of the radiometer between the signal channel and the reference channel. Gain variations in the radiometer can have their effects neutralized by measuring the difference in signals between the antenna signal and the noise input of the resistor (reference channel). The output of the Dicke switch 110 is then amplified by an amplification stage 112, including, for example four or more LNA amplifiers.
In a typical case, the difference signal is obtained by using a synchronous (differential) detector circuit 116. A square waveform that is used to switch the radiometer input from antenna to resistor is also used to drive the synchronous detector. The switching rate is typically between 30 and 10000 Hz. An analog read out circuit 118, as apart of the radiometer or separate from the radiometer, reads the output of the differential circuit and generates a square signal, as shown.
By modulating between the antenna signal channel and the reference signal channel, the (Dicke switched) radiometer also removes temperature drifts and provides a fully calibrated signal to a display. However, this radiometer requires the use of a band pass filter 114 to match the energy received from the antenna element and the reference channel. Without this matching, the reference channel will swamp out the signal channel resulting in catastrophic errors during the subtraction process of the signal and reference channels in the back end. Accordingly, a band pass filter 116 is placed after the Dicke switch (and a third LNA stage 112), which adds losses to the system that cannot be recovered. Additionally, the band pass filter could limit the signal chain gain, thereby reducing the sensitivity of the radiometer chip.
The operation of a Dicke switched radiometer reties upon comparison of an incident (or object) signal path and a reference signal path such that the object signal is received with minimal added noise arising from the systems own noise (random signal fluctuations). Since Dicke switched radiometers use frequency independent noise sources having a constant (white) noise spectrum, noise signals can easily be introduced outside an intended signal bandwidth.
The bandpass filter 114 is used to filter out this white noise source outside of the Dicke switch modulation signal. However, this bandpass filter 114 negatively impacts system performance since in-band attenuation of a bandpass filter reduces desired signal strength by narrowing the bandwidth of the amplification stage/chain 112. The signal bandwidth directly corresponds to how much power is collected the detector 116. Any reduction in bandwidth lowers the amount of signal collected and negatively impact the performance of the radiometer.
Some conventional designs use a variable cold/warm noise source that allows for improved calibration accuracy by allowing the reference path noise temperature to automatically adjust for optimal performance. A cold/warm noise source also uses impedance matching circuits to couple noise power to a radiometer. However, this design does not attempt to match frequency response with the antenna path therefore requiring use of a bandpass filter after the Dicke-switch.
Accordingly, there is a need for an improved Dicke switch radiometer that has better frequency response characteristics.